Frequency converter



June 24, 1941. J. FARRINGTQN Re. 21,839 FREQUENCY CONVERTER Original Filed March 16, 1938 INVENTOR y F. FARRINGT N B ATTORNEY Reissued June 24, 1941 UNITED STATES PATENT OFFICE FREQUENCY CONVERTER John F. Farrington, Tilton, N. H., assignor to Hazeltine Corporation, a corporation of Delaware OriginalNo.2,166,522, dated July 18, 1939, Serial No. 196,124, March 16, 1938. Application forre- -issue April 10, 1941, Serial No. 387,991

11 Claims.

This invention :relates generally to frequency converters, and particularly to frequency converters adapted to be utilized in modulated-carrier signal receivers of the superheterodyne type.

In frequency converters ofsuperheterodyne receivers, there is provided a local oscillator with a frequency-determining circuit for generating oscillations which are modulated with a: received signal to produce an intermediate-frequency signal. 'lhereiszatendency-for themodulator to 'affect the frequency-0f the local oscillations. This variation of the oscillator'frequency due'to the fact'that themodulator is usually coupled relatively closely to the frequency-determining cir-- cult of the oscillator so that changes in the imn pedance of the modulator grid circuit to which the oscillator i coupled affect the natural resonant frequency of the frequency-determining cir cuit. Such changes in impedance maybe caused by variations in the supply potentials or A. V. C. potential applied to the modulator. Variouswattempts have been madeto'eliminate this difiiculty. One such arrangement utilizes a common cathode resistor for coupling the'oscillator section and the modulator section of a'frequency converter.

With thi arrangement the effect of variations in impedance or trans conductance .of the modulator section of the frequency converter upon the frequency of the oscillator ismaterially reduced. However, when this circuit is utilizedin a superheterodyne receiver, the efficiency of the converter is reduced by degeneration due to the cathode resistor which decreases 'the'signal amplitude applied to the input electrodes o-f the-modulator and-which also applies to the input electrodes intermediate-frequency waves in such phase as to reduce the.intermediate-frequency output.

- It is an object of the present invention, therefore, to provide a frequencychangersuitable for use in superheterodyne receivers in which the oscillation frequency is substantially independent of .the impedance and ,transconductance characteristicsof the modulator. 1

It is "another object of the inventionto provide ,a frequency converter suitable for use-in superheterodyne receivers in which the modulator is not substantially coupled with th 'frequency-determining circuit of the oscillator. vIn accordance with the invention, a 'frequenc I converter comprises, a modulatorrsection and an oscillator section, each of the sections including ia space currentpathcomprising input and output electrodes. A signaleinputcircuit is coupled to the modulator input-e1ectrode-andea frequencydetermining circuit .is coupled to the oscillator section, impedance means being included in the space current path of the oscillator section and substantially uncoupled with the above-mentioned frequency-determiningcircuit. The converter comprises means for coupling the oscillation voltage developed across the above-mentioned impedance means to an input electrode in the modulator section to modulate the signal input -to the modulator section, whereby the resonant frequency of the frequency-determining circuit is not appreciably affected by variations in the impedance of the circuit of the modulator electrode to which said oscillation voltage is coupled,

Referring now to-the drawing, Figs. 1 and 2 are circuit diagrams, partly schemati of complete .superheterodyne receivers embodying different forms of the invention in the frequency converter or flrst'detector of the receiver.

Referring now more particularly to Fig. 1, there is shownschematically a complete superheterodyne radio receiver embodying the present invention in a preferred form. Ingeneral, the receiver includes a radio-frequency amplifier HI having its input circuit connected to an antenna I land ground 12 and its output circuit connected .to a frequency changer or oscillator-modulator I3. Connected in cascade with the frequency changer I3, in the order named, are an intermediate-frequency amplifier M of one or more stages, adetector and automatic amplification control supply 15, an audio-frequency amplifier 16 of one'or'more stages, and asound reproducer .ll. An-automatic amplification control bias, de-

rived from A. V. C. supply I5, is applied to the grids of one or more of the'tubes of amplifier l0, oscillator-modulator l3, and one or'more of the gridsof intermediate-frequencyamplifier M in order to maintain the signal input to detector l5 within a relatively narrow range for a wide range Qof received signal amplitudes. ,stood that the various circuits just described, with It will be underthe exception of frequency-converter .circuit I3, may be of a conventional construction and operation, the details of which are well known in'the art, rendering detailed description thereof unnecessary;

Considering briefly the operation of the receiveras a whole and neglecting for the moment the operation of frequency converter I3, per se,

1 presently tobe described, a desired modulated signal is selected and amplified by radio-frequency amplifier H); converted tov a modulated intermediate-frequency signal in :frequency changer l3, amplified by intermediate-frequency amplifier l4, and rectified by the detector l5, thereby deriving the audio-frequencies of modulation and the automatic amplification control bias potential. The audio-frequencies of modulation are, in turn, amplified in audio-frequency amplifier I6 and are reproduced by sound reproducer I1. The automatic amplification control circuit serves to maintain the volume output of the receiver within a relatively narrow range for a wide range of received signal amplitudes.

Referring now more particularly to the parts of the system involving the present invention, frequency converter I3 comprises a modulator section 20 and an oscillator section 2|, The modulator section 20 comprises a mixer or modulator vacuum tube 22 having an input circuit coupled to radio-frequency amplifier l and an output circuit coupled to intermediate-frequency amplifier M. The oscillator section 2! comprises a vacuum tube 23; a frequency-determining circuit connected to the grid circuit of tube 23 and including a variable tuning condenser 24 and an inductance 25; and an oscillation feed-back circuit comprising inductance 26 coupled to inductance 25 and connected to the anode of tube 23 through a blocking condenser 21. The tube 23 includes an un-bypassed resistor 28 in its cathode-ground circuit. Resistor 28 and condenser 29 serve to couple oscillator 2| to an input grid of tube 22, preferably the second input or injector grid. Suitable operating potentials are provided for tubes 22 and 23 from sources indicated on the drawing as C, +Sc, and +3 In considering the operation of the circuit just described, it will be seen that the oscillator section 2| is a conventional one, generating local oscillations at the resonant frequency of the frequency-determining circuit 24, 25. The variations in the plate current of tube 23 generate voltage waves of oscillator frequency across resistor 28. These waves are applied through condenser 29 to the injector grid of modulator tube 22 and they modulate the received signals at this frequency. It will be seen that resistor 28 is not substantially coupled with the oscillator frequency-determining circuit 24, 25. Furthermore, the value of resistor 28 can be made low as compared to the impedance of the injector grid circuit of tube 22, which is effectively in parallel with it. Consequently, variations in the impedance of the injector grid circuit, caused by changes in the operating potentials applied to the modulator tube 22, do not materially affect the frequency of the oscillator frequency-determining circuit 24, 25, or the amplitude of the oscillator voltage applied to the iniector grid.

As illustrative of a specific application utilizing the circuit of Fig, 1, the following circuit constants are given:

Tube 22 Type 6L7 Tube 23 Type 605 Resistor 28 ohms 1,000 Resistor 28' ohms 50,000

In Fig. 2 is shown a receiver embodying another form of the invention in which the space current paths of the oscillator and modulator sections are included in a common envelope, comprising a duplex vacuum tube 40. The circuit of the receiver of Fig. 2 is generally similar to that of Fig. l and like elements in the two figures have been given identical reference numerals. The duplex vacuum tube 40 contains modulator and oscillator sections which have a common cathode. The input circuit of the modulator section iscoupled to the output circuit of radio-frequency amplifier l0 and the output circuit of the modulator section is coupled to the input circuit of intermediate-frequency amplifier M. The oscillator circuit is similar to Fig. 1 except that resistor 28 is omitted from the cathode circuit and replaced by resistors 45 and 46 in the space current path of the oscillator section of tube 46. Resistor 45 and condenser 41 constitute the means for coupling the oscillator to an input grid of the modulator section of vacuum tube 48.

The operation of the circuit of Fig, 2 will be apparent from the description given above with respect to the operation of the circuit of Fig. 1. It is seen that oscillation voltages are developed across resistor 45 due to variations in the space current of the modulator section. Resistor 45 is not substantially coupled to the frequency-determining circuit due to the decoupling resistor 46. Therefore, variations in the impedance of the modulator section of vacuum tube or the transconductance thereof do not materially affect the resonant frequency of frequency-determining circuit 24, 25.

It will be understood that either of the coupling circuits, comprising elements 28, 2B, and 29 of Fig. 1 or 45, and 41 of Fig. 2, may be designed to have a frequency-response characteristic which varies over the operating range of the oscillator in a complementary manner to the voltage developed across the output impedance in the space current path of the oscillator, thereby to maintain the voltage supplied to the modulator section from the oscillator section substantially constant,

While in each of the circuits illustrated and specifically described the locally-generated oscillations are applied to the third grid of the frequency converter tube, it will be understood by those skilled in the art that such oscillations may be applied to any suitable grid of such tube in accordance with well-known practice.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes, a signalinput circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section, impedance means included in the space current path of said oscillator section and substantially uncoupled with said frequency-determining circuit, a second input electrode in the space current path of said modulator section, and means for coupling an oscillation voltage developed across said impedance to said second input electrode to modulate the signal input to said modulator section, whereby the resonant frequency of said frequencydetermining circuit is not appreciably affected by variations in the impedance of the circuit of said second electrode.

2. A frequency converter comprising a modulator section andan oscillator section, each of said sections including a space current path comprising input and output electrodes, 'a signalinput circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section, a resistor included in the space current path of said oscillator section and substantially uncoupled with said frequency-determining circuit, a second input electrode in the space current path of said modulator section, and means for coupling an oscillation voltage developed across said resistor to said second input electrode to modulatethe signal input to said modulator section, whereby the resonant frequency of said frequency-determining circuit is not appreciably aifected by variations in the impedance of the circuit of said second electrode.

3. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes, a signalinput circuit coupled to the modulator input electrode, a frequency-determining circuit included in the input circuit of said oscillator section, impedance means included in the space current path of said oscillator section and substantially uncoupled with said frequency-determining circuit, a second input electrode in the space current path of said modulator section, and means for coupling an oscillation voltage developed across said impedance to said second input electrode to modulate the signal input tosaid modulator section, whereby the frequency of said frequency-determining circuit is not appreciably affected by variations in the impedance of the circuit of said second electrode.

4. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes and separate cathodes, a signal-input circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section, an unbypassed resistor included in the cathode circuit of said oscillator section and substantially uncoupled with said frequency-determining circuit, a second input electrode in the space current path of said modulator section, and means flor coupling an oscillation voltage developed across said resistor to said second input electrode to modulate the signal input to said modulator section, whereby the resonant frequency of said frequency-determining circuit is not appreciably affected by variations in the impedance of the circuit of said second electrode.

' 5. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes, said paths including a common cathode circuit, a signal-' input circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section, impedance means included in the space current path individual to said oscillator section and substantially uncoupled with said frequency-determining circuit, a second input electrode in the space current path of said modulator section, and means for coupling an oscillation voltage developed across said impedance to said second input electrode to modulate the signal input to said modulator section, whereby the resonant frequency of said frequency determining circuit is not appreciably affected by variations in the impedance of the circuit of said second electrode.

6. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes, said paths including a common cathode circuit, a signal-input circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section, a first resistor included in the space current path individual to said oscillator section, a second resistor for substantially decoupling said first resistor and said frequencydetermining circuit, a second input electrode in the space current path of said modulator section, and means for coupling an oscillation voltage developed across said first resistor to said second input electrode to modulate the signal input to said modulator section, whereby the resonant frequency of said frequency-determining circuit is'not appreciably affected by variations in the impedance of the circuit of said second electrode.

7. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes, a signal-input circuit coupled to the modulatorinput electrode, a tunable frequency-determining circuit coupled to said oscillator section, a second input electrode in the space current path of said modulator section, impedance means included in the space current path of said oscillator section and coupled to said second input electrode for coupling a voltage to said second electrode to modulate the input signal to said modulator section, said impedance means being substantially uncoupled with said frequency-determining circuit, whereby the resonant frequency of said frequency-determining circuit is not substantially affected by variations in the impedance of the circuit of said second electrode, said impedance and its coupling to said second input electrode being so proportioned that the voltage coupled to said second input electrode from said impedance means is substantially constant over the turiing range of said frequency-determining circui 8. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes, a signal-input circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section, impedance means included in the space current path of said oscillator section and substantially uncoupled with said frequency-determining circuit, and means for coupling an oscillation voltage developed across said impedance to an input electrode in said modulator section to modulate the signal input to said modulator section, whereby the resonant frequency of said frequency-determining circuit is not appreciably affected by variations in the impedance of the circuit of the modulator electrode to which said oscillation voltage is coupled.

9. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes and separate cathodes, a signal-input circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section,

an unby-passed resistor included in the cathode circuit of said oscillator section and substantially uncoupled with said frequency-determining circuit, and means for coupling an oscillation voltage developed across said resistor to an input electrode of said modulator section to modulate the signal input to said modulator section, whereby the resonant frequency of said frequency-determining circuit is not appreciably afiected by variations in the impedance of the circuit of the modulator electrode to which said oscillation voltage is coupled.

10. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes, said paths including a common cathode circuit, a signal-input circuit coupled to the modulator input electrode, a frequency-determining circuit coupled to said oscillator section, impedance means included in the space current path individual to said oscillator section and substantially uncoupled with said frequencydetermining circuit, and means for coupling an oscillation voltage developed across said impedance to an input electrode of said modulator section to modulate the signal input to said modulator section, whereby the resonant frequency of said frequency-determining circuit is not appreciably affected by variations in the impedance of the circuit of the modulator electrode to which said oscillation is coupled.

11. A frequency converter comprising a modulator section and an oscillator section, each of said sections including a space current path comprising input and output electrodes, a signalinput circuit. coupled to the modulator-input electrode, a tunable frequency-determining circuit coupled to said oscillator section, impedance means included in the space current path of said oscillator section and coupled to an input electrode of said modulator section for coupling a voltage thereto to modulate the input signal to said modulator section, said impedance means being substantially uncoupled with said irequency-determining circuit, whereby the resonant frequency of said frequency-determining circuit is not substantially affected by variations in the impedance of the circuit of the modulator electrode to which said oscillation voltage is coupled, said impedance and its coupling to said modulator section being so proportioned that the voltage coupled to said modulator section from said impedance means is substantially constant over the tuning range of said frequencydetermining circuit.

- JOHN F. FARRINGTON. 

